XP still has an edge as most of the current games were designed to run on it, using DirectX 9, and nVidia, ATI, and the others had years to tweak the drivers for maximum performance. None of that is the case with Vista, so most are finding XP still gives gamers the edge. Thus, if you are looking to give him the best gaming performance, XP is still the way to go.

As to x64 vs x86, driver and security software compatibility are the follies of 64-bit computing. Actual performance can vary, just as when comparing single core processors to dual core models, neither claiming a unanimous victory. The key benefit to 64-bit versions of Windows is the elimination of the 4GB barrier when it comes to RAM, but that shouldn't apply in your case since you're only installing to 2GB. Thus, the choice is yours, though I would take x86 to avoid potential driver issues.

Hope this helps,